The present invention relates to a multilayer resonance device for use in a device utilizing a magneto-optical effect and to a magneto-optical recording medium for recording/reproducing information through a magneto-optical recording/reproducing operation.
In the recent PC (personal computer) environment for dealing with dynamic image data, a magneto-optical recording medium, which has been widely used as a data recording device for a computer, is required to have a larger recording capacity, namely, there is a demand for a magneto-optical disk capable of high density recording. In order to increase a packing density, it is necessary to form short recording marks, but such short recording marks decrease a reproducing signal output due to waveform interference between the marks. Therefore, as a reproducing method for obtaining a reproducing signal without causing the waveform interference between the marks, various types of magnetically induced super resolution technique have been proposed. However, a magnetically induced super resolution medium has a structure including stacked layers of plural magnetic layers respectively having different magnetic characteristics, and hence, it is very difficult to control the magnetic characteristic and the exchange coupled force among the magnetic layers. Therefore, mass-production of such a medium is difficult. In addition, it is disadvantageously necessary to set recording/reproducing apparatus in accordance with the layer structure of the medium.
Furthermore, in a general magneto-optical disk, for example, in a magneto-optical disk having a structure including a SiN layer, a TbFeCo layer, another SiN layer and an aluminum layer successively stacked in this order on a substrate, an incident laser beam is made not to be reflected within the layers in order to attain a large magneto-optical effect. However, even in a magneto-optical disk using this non-reflection condition, it is impossible to obtain a Kerr rotation angle larger than one degree, and hence, such a disk is not suitable to reproducing short recording marks. Moreover, in a magneto-optical disk utilizing optics using evanescent light, that is, so-called near-filed optics, a reproducing signal output is so small that a medium for attaining a larger magneto-optical effect is demanded.
On the other hand, in the field of an optical isolator, a garnet monocrystal layer including YIG (yttrium-iron-garnet) as a main component is used, but the use is limited because light loss cannot be avoided. As a countermeasure, the present inventors and applicant have found that an optical isolator having a multilayer structure formed by alternately stacking a garnet monocrystal layer including YIG as a main component and a dielectric substance can attain a large magneto-optical effect and a high transmittance and can be free from light loss. Also, as a result of development of this finding, the present inventors and applicant have proposed a magneto-optical multilayer layer in Japanese Patent Application Laid-Open No. 10-54965 (1998)
The magneto-optical multilayer layer disclosed in Japanese Patent Application Laid-Open No. 10-54965 (1998) has a structure in which a magnetic substance and a dielectric substance are alternately stacked with their thicknesses irregularized, and can attain a very large magneto-optical effect. However, the number of stacked layers is large, and hence, the total thickness is large. Therefore, this magneto-optical multilayer layer is disadvantageously difficult to practically apply to a device utilizing a magneto-optical effect such as a microcavity and the aforementioned magneto-optical recording medium.
The present invention was devised to overcome the aforementioned problems. One object of the invention is providing a multilayer resonance device which can attain a large magneto-optical effect with a practical thickness by disposing regularly stacked layers each formed by alternately stacking a magnetic substance and a dielectric substance with thickness regularity, and an irregular layer including a magnetic substance and having a thickness disaccording with the thickness regularity to be sandwiched therebetween.
Another object of the invention is providing a multilayer resonance device which can attain a large magneto-optical effect with a practical thickness by disposing regularly stacked layers each formed by alternately stacking different dielectric substances with thickness regularity, and an irregular layer including a magnetic substance and having a thickness disaccording with the thickness regularity to be sandwiched therebetween.
Still another object of the invention is providing a magneto-optical recording medium capable of high output reproduction even when a light beam with a short wavelength is used by disposing the aforementioned multilayer resonance device on a substrate.
The multilayer resonance device of this invention comprises two regularly stacked layers each formed by alternately stacking a magnetic substance and a dielectric substance with thickness regularity; and an irregular layer including at least a magnetic substance, disposed between the regularly stacked layers and having a thickness disaccording with the thickness regularity.
Thus, reflection layers each formed by alternately stacking the magnetic substance and the dielectric substance are disposed so as to sandwich the magnetic substance, and hence, a large rotation of a polarization plane can be attained.
Alternatively, the multilayer resonance device of this invention comprises two regularly stacked layers each formed by alternately stacking different dielectric substances with thickness regularity; and an irregular layer including at least a magnetic substance, disposed between the regularly stacked layers and having a thickness disaccording with the thickness regularity.
Thus, the dielectric substances are alternately stacked, and hence, a multilayer layer working as a reflection layer can be easily formed.
In the multilayer resonance device, the regularly stacked layers together have a stacked structure symmetrical with respect to the irregular layer.
Since the layer structure is symmetrical with respect to the irregular layer, the resonance effect can be enhanced.
Alternatively, the multilayer resonance device of this invention comprises first and second optical layers having different optical characteristics and respectively having first and second thicknesses determined in accordance with the optical characteristics thereof, which are alternately stacked to form a multilayer stacked layer; and a magnetic layer disposed substantially at the center of the multilayer stacked layer and having a thickness different from the first and second thicknesses.
Thus, the optical layers having peculiar optical characteristics, such as a magnetic substance and a dielectric substance, are stacked into a predetermined thickness. Therefore, an interference layer in which light is localized at the center is formed, so that the magnetic layer disposed substantially at the center of the multilayer resonance layers can exhibit a large magneto-optical effect.
The magneto-optical recording medium of this invention comprises a first regularly stacked layer formed by alternately stacking different dielectric substances with thickness regularity; a magnetic layers having a thickness disaccording with the thickness regularity, for recording and storing information; and a second regularly stacked layer with the same thickness regularity as the first regularly stacked layer, wherein the first regularly stacked layer, the magnetic layer and the second regularly stacked layer are stacked in this order.
Accordingly, a light beam irradiating through a substrate is resonated between the first regularly stacked layer, the magnetic layer and the second regularly stacked layer, so that the magnetic layer for recording and storing information can attain a large magneto-optical effect.
In the magneto-optical recording medium, the first regularly stacked layer and the second regularly stacked layer together have a stacked structure symmetrical with respect to the magnetic layer.
Accordingly, since the layer structure is symmetrical with respect to the magnetic layer for recording and storing information, the resonance effect can be enhanced.
In the magneto-optical recording medium, the magnetic layer includes rare earthxe2x80x94transition metal.
Thus, since the rare earthxe2x80x94transition metal such as TbFeCo and DyFeCo is used in the magnetic layer, the formation of the layer can be eased.
In the magneto-optical recording medium, a reflection layer including a metal is formed on the first or second regularly stacked layer on a surface not in contact with the magnetic layer.
Thus, since the reflection layer including the metal is provided, the temperature distribution on the medium can be controlled.
In the magneto-optical recording medium, a recording mark with a dimension smaller than a spot diameter of the light beam is formed in the magnetic layer.
Accordingly, since a light beam having a specific wavelength is resonated in accordance with a temperature, the magneto-optical effect can be enhanced merely in a predetermined temperature area in the temperature distribution caused by irradiation with the light beam in a reproducing operation. Therefore, a large magneto-optical effect can be attained in a part of the beam spot, and a recording mark smaller than the spot diameter can be read. In this manner, the super resolution reproduction can be conducted, resulting in realizing high density recording.
Alternatively, the magneto-optical recording medium of this invention comprises the aforementioned multilayer resonance device.
Accordingly, information is recorded in the irregular layer or the magnetic layer disposed substantially at the center. As a result, a large magneto-optical effect can be attained and a large reproducing output can be obtained.
Another object of the invention is providing a magneto-optical recording medium that can exhibit a further larger magneto-optical effect. Still another object of the invention is providing a method of reproducing a magneto-optical recording medium in which information recorded in a high density can be reproduced by dividing a reproducing signal into three or more levels in reproducing the magneto-optical recording medium. Furthermore, still another object of the invention is providing a method of reproducing a magneto-optical recording medium in which information can be reproduced with higher output level by obtaining a rotation angle of a polarization plane of light irradiating the magneto-optical recording medium to be used as a reproducing signal.
In the magneto-optical recording medium of this invention, each of the first regularly stacked layer and the second regularly stacked layer includes three or more pairs of the different dielectric substances alternately stacked.
Accordingly, since each of the first and second regularly stacked layers is formed by stacking three or more pairs of dielectric substances, a further larger magneto-optical effect can be attained, resulting in obtaining high reproducing output level.
In the magneto-optical recording medium of this invention, the number of pairs of the different dielectric substances alternately stacked in the first regularly stacked layer is different from the number of pairs of the different dielectric substances alternately stacked in the second regularly stacked layer.
Accordingly, since the numbers of pairs of the dielectric substances are different in the first and second regularly stacked layers, the regularly stacked layer including a larger number of pairs has a higher reflectance than the other regularly stacked layer including a smaller number of pairs. Therefore, when the reproducing light irradiates a face of the magneto-optical recording medium closer to the regularly stacked layer including a smaller number of pairs, the reflectance of the entire resonance layer can be improved without degrading the magneto-optical effect.
In the magneto-optical recording medium of this invention, the numbers of the stacked dielectric substances in the first regularly stacked layer and the second regularly stacked layer are different from each other.
Accordingly, since the numbers of stacked dielectric substances are different in the first and second regularly stacked layers, the regularly stacked layer including a larger number of stacked dielectric substances has a larger reflectance than the other regularly stacked layer including a smaller number of stacked dielectric substances. Therefore, when the reproducing light irradiates a face of the magneto-optical recording medium closer to the regularly stacked layer including a smaller number of stacked dielectric substances, the reflectance of the entire resonance layer can be improved without degrading the magneto-optical effect.
In the magneto-optical recording medium of this invention, SiO2 and SiN are used as the dielectric substances.
Accordingly, when the magnetic layer is formed out of, for example, a rare earth transition metal amorphous alloy, the magnetic layer can be protected from degradation such as oxidation by using a SiN film with a superior protecting effect as the dielectric substance.
In the magneto-optical recording medium of this invention, each of the dielectric substances has a thickness defined on the basis of a wavelength of light used in optical reproduction and a refractive index of the dielectric substance as follows:
t less than xcex/(4M)
wherein t indicates the thickness of the dielectric substance, xcex indicates the wavelength of the light and M indicates the refractive index of the dielectric substance.
Since the optical length of the center portion of the resonance layer is shifted to be longer than the resonance wavelength, the actual resonance wavelength is shifted to be longer than the theoretical resonance wavelength. Therefore, when the dielectric substance has a thickness smaller than xcex/(4M), the thickness according with the actual resonance wavelength can be attained.
In the method of reproducing a magneto-optical recording medium of this invention, information recorded in the magnetic layer is reproduced by irradiating a face of the magneto-optical recording medium, with a light beam, closer to either the first or the second regularly stacked layer including a smaller number of pairs of dielectric substances or a smaller number of stacked dielectric substances.
Accordingly, since the numbers of pairs of the dielectric substances or the numbers of stacked dielectric substances included in the first and second regularly stacked layers are different from each other, the regularly stacked layer including a larger number of pairs of the dielectric substances or a larger number of stacked dielectric substances has a larger reflectance than the other regularly stacked layer. Therefore, when the reproducing light irradiates a face of the magneto-optical recording medium closer to the regularly stacked layer including a smaller number of pairs of dielectric substances or a smaller number of stacked dielectric substances, the reflectance of the entire resonance layer can be improved without degrading the magneto-optical effect.
The method of reproducing a magneto-optical recording medium of this invention comprises the steps of obtaining a reproducing signal on the basis of a beam light reflected on the magneto-optical recording medium; dividing the obtained reproducing signal into three or more levels; and reproducing information on the basis of the reproducing signal by using previously obtained information patterns corresponding to the respective levels.
Since each of the three or more levels obtained by dividing the reproducing signal corresponds to an information pattern of plural bits, information of plural bits can be reproduced from a reproducing signal, for example, corresponding to one spot area of the reproducing light. Accordingly, information recorded in a high density by using a recording mark smaller than the spot of the reproducing light can be accurately reproduced.
The method of reproducing a magneto-optical recording medium of this invention comprises the steps of receiving a light beam reflected by the magneto-optical recording medium; detecting a rotation angle of a polarization plane of the received light beam; and reproducing information by using the detected rotation angle as a reproducing signal.
Also, in the method of reproducing a magneto-optical recording medium of this invention, in the step of receiving the reflected light beam, the reflected light beam having passed through an analyzer rotatable about a light transmission axis is received, and in the step of detecting the rotation angle of the polarization plane, the rotation angle of the polarization plane is detected on the basis of a rotation angle of the analyzer.
Moreover, in the method of reproducing a magneto-optical recording medium of this invention, in the step of receiving the reflected light beam, the reflected light beam having passed through an analyzer invertible about a light transmission axis in a predetermined angle range is received, and in the step of detecting the rotation angle of the polarization plane, the rotation angle of the polarization plane is detected on the basis of a rotation angle of the analyzer.
The rotation angle of the polarization plane of the light reflected by the magneto-optical recording medium, namely, the Kerr rotation angle, is directly detected by, for example, rotating or inverting the analyzer. Thus, there is no need to detect the intensity variation of the signal. Accordingly, for example, even when the reflectance of the magneto-optical recording medium is not very high, information can be accurately reproduced because the Kerr rotation angle is large.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.